1. Field of the Invention:
The present invention relates to an optical pickup having a driving unit for driving a driven member having an objective lens by a magnetic force.
2. Description of the Prior Art:
A conventional optical pickup shown in FIGS. 1 to 3 is known as an optical pickup of this type.
A bearing 2 is fitted through along an axis of a cylindrical objective lens holder 1 and is held therein. The bearing 2 is fitted around a support shaft 4, and the lower end of the support shaft 4 is fixed on a disk-like base 3 made of a soft magnetic material.
An objective lens 6 is held in the objective lens holder 1 at a position spaced apart from an axis of the support shaft 4 of the objective lens holder 1 through a lens frame 5 in such a manner that an optical axis of the objective lens 6 is substantially parallel to the axis of the support shaft 4.
A focus coil 7 is wound around a wall 1a of the objective lens holder 1 so as to linearly drive the objective lens holder 1 toward an in-focus position. Four tracking coils 8A to 8D are adhered to the outer surface of the focus coil 7 in an overlapping manner so as to rotatably drive the objective lens 6 in the tracking direction. The tracking coils 8A to 8D have a rectangular shape and are adhered to the focus coil 7 on the objective lens holder 1 at equal angular intervals.
A pair of inner yokes 3b and 3c are formed integrally on an upper surface 3a of the base 3 at the central portion of the base 3 so as to oppose each other through the support shaft 4. A magnetic ring 9 and a pair of outer yokes 10a and 10b in contact with the magnetic ring 9 are mounted on the base 3 so as to surround the inner yokes 3b and 3c. More specifically, the outer yokes 10a and 10b are formed integrally with each other and oppose the inner yokes 3b and 3c through the wall 1a of the objective lens holder 1.
A magnetic flux is formed perpendicular to the wall 1a of the objective lens holder 1 in a space between the inner yokes 3b and 3c and the outer yokes 10a and 10b. A drive current flows through the focus coil 7 to linearly drive the objective lens holder 1 along the axis of the support shaft 4 which corresponds to the focusing direction. A drive current flows through the tracking coils 8A to 8D to rotate the objective lens holder 1 around the shaft 4 along the tracking direction.
A rubber damper member 11 serves as a neutral position holding mechanism for tracking the objective lens holder 1. Reference numeral 12 denotes a connecting pin.
In the optical pickup having the construction described above, the space between the inner yokes 3b and 3c and the outer yokes 10a and 10b serves as a space for generating a magnetic field for driving the objective lens 6. Even if a magnetic field of high magnetic flux density is obtained to effectively drive the objective lens 6 with a small drive current, the optical pickup has a closed loop mechanism requiring the inner yokes 3b and 3c and the outer yokes 10a and 10b. Therefore, the optical pickup must have a large size due to the inclusion of the yokes 3b, 3c, 10a and 10b. In addition to this disadvantage, manufacture of the yokes 3b, 3c, 10a and 10b results in a high cost and a complicated assembly.